Ionizing radiation therapy is a therapeutic method involving irradiating a lesion with radiation to kill cells in the lesion. This therapy is used for not only malignant tumors but also some benign diseases, for example, in improving a keloid and treating thyroid ophthalmopathy. The ionizing radiation therapy is one of the major therapeutic methods for malignant tumors, but known to have a problem of an adverse effect of damaging normal organs and tissues surrounding the lesion by exposure. In order to reduce such adverse effect, efforts have been made to avoid exposure of normal tissues. For example, there has been offered a method involving irradiating a lesion from a close distance with a high-energy electron beam that does not reach a deep portion, such as a particle beam, thereby irradiating the lesion with a high dose of radiation at one time without any adverse effect, which is impossible with extracorporeal irradiation. It can be said that a therapeutic method based on the particle beam irradiation is excellent in dose concentration, sharpness of a dose distribution boundary, biological effect (killing effect), effect on a hypoxic cancer, effect on a radioresistant cancer, and the like as compared to X-ray irradiation. However, even in the case of the particle beam irradiation, exposure of any normal tissue should be avoided as much as possible.
As a method of avoiding exposure of a normal tissue, there is given, for example, a method involving, in irradiation of a lesion with radiation, performing highly accurate positioning of a target and three-dimensionally concentrating a high-energy beam on the resultant position with high precision (three-dimensional irradiation method). In addition, there has also been adopted a method of avoiding exposure of a normal tissue involving fixing a site to be irradiated from the outside with, for example, various mechanical fixtures and shells made of plastics to reduce the movement of the target. However, movements of organs include vital ones such as a respiratory motion and a heartbeat, and hence should not be regulated completely.
As means for solving the problems, there has been proposed an artificial material which is disposed between a lesion and an adjacent normal tissue to separate the normal tissue to a position with no risk of exposure (spacer). As such spacer for ionizing radiation therapy (hereinafter, sometimes simply referred to as “spacer”), there has been known an approach of using a medical material produced by combining a film obtained by subjecting polytetrafluoroethylene to stretch processing and a polyurethane polymer, such as a GORE-TEX™ (Non Patent Literatures 1 and 2). The GORE-TEX™ is a medical material and has assured safety for use in a living body. However, the spacer is removed by performing abdominal surgery again after the completion of ionizing radiation therapy in order to eliminate a problem that may arise if foreign matter is left in the body.
However, repeated abdominal surgery involves a burden on and a risk to a patient, and when the spacer and a normal tissue adhere to each other, an adhesiotomy or the like is necessary, resulting in a further increased burden on and risk to the patient. As means for solving this problem, Patent Literature 1 and Patent Literature 2 each propose a spacer that does not need to be removed after therapy. The implant (spacer) disclosed in Patent Literature 1 contains a biocompatible material implantable with a tube, a hose, an injection needle, or the like in a non-open surgical manner, has a viscosity at 37° C. before implantation in the range of 20 to 3,000 mPa·s, and shows an increase in viscosity after implantation as compared to before implantation. As a result of containing the biocompatible material, the spacer does not require abdominal surgery for removing the implant after therapy. In addition, when a biodegradable material is used for the biocompatible material, the implant does not remain in the body because the material is naturally degraded in the living body.
The sponge (spacer) for radiation therapy disclosed in Patent Literature 2 is constructed of collagen having an intermolecular cross-link or a mixture of collagen and gelatin. The sponge has compression recoverability, and hence separation of a normal tissue can be performed by introducing the sponge in a compressed state into the intraperitoneal cavity with a tracker or the like, and recovering the sponge to its original form through utilization of intraperitoneal water or the like. Further, the sponge is insoluble in water at an early stage after its introduction into the intraperitoneal cavity, but is absorbed in the body as time passes. Therefore, there is no need for abdominal surgery for removing the sponge after therapy.